The importance of automatic identification systems is growing, particularly in the service sector, and in the fields of logistics, commerce and industrial production. Automatic identification systems are thus implemented more and more in these and other fields and will probably substitute barcode systems in the future. Further applications of identification systems relate to the identification of persons and animals. It is also very interesting to have the possibility of monitoring and controlling a cooling history of food, particularly of perishable food, by using an identification system.
In particular, contactless identification systems such as transponder systems are suitable for wireless transmission of data in a fast manner and without cable connections, which may be disturbing. Such systems use the emission and absorption of electromagnetic waves, particularly in the high-frequency domain. A transponder may be realized as an RFID tag (“radio frequency identification tag”) or as a smart card.
In a transponder according to the prior art, the coil supply voltage may be limited to a certain value, thus compensating different field strengths between a reader station and the transponder. For proper performance, particularly at a maximum reading distance, this value may be set to a minimum limit at which a demodulator of the transponder starts operating properly.
According to the prior art, contactless RFID tags comprise a coil communicating via an electromagnetic high-frequency field, wherein the coil voltage may be limited to a particular value by a parallel voltage limiter circuit. This voltage limit is used when the RFID tag receives commands or data from a read/write device, and for load modulation, i.e. when the RFID tag transmits data to a read/write device.
An RFID tag 100 according to the prior art will be described hereinafter with reference to FIG. 1.
Strictly speaking, FIG. 1 only shows the front-end circuitry of an RFID tag, wherein other portions of an RFID tag are omitted. However, the term “RFID tag” may hereinafter be used as a short form for “front-end circuitry of an RFID tag”. The front-end circuitry is indicated in FIG. 1 by a broken-line box.
The RFID tag 100 comprises an antenna coil 101 adapted to receive electromagnetic waves, particularly in the high-frequency domain, emitted by a read/write device (not shown). When the antenna coil 101 absorbs electromagnetic radiation, a voltage is generated between a first coil connection 111 and a second coil connection 112. Together with a capacitor 113, the antenna coil 101 defines a range of wavelengths of electromagnetic radiation which the RFID tag 100 may absorb.
Four rectifier diodes 102 are provided between the antenna coil 101 and an antenna voltage limiter circuit 104 and are connected to rectify a voltage of the antenna coil 101. Two of the rectifier diodes 102 are connected to a reference or ground potential 103, while the other two rectifier diodes 102 form the supply for the antenna voltage limiter circuit 104.
The rectified voltage is supplied to an input of the regulator unit 105 as the currently prevailing value. The regulator unit 105 receives, as a further input, a constant reference voltage from a reference voltage unit 106 and generates a regulating signal Vreg on the basis of a comparison of the current value and the constant target value provided by the reference voltage unit 106.
This regulating signal Vreg is supplied to a receiving demodulator unit 107 for demodulating a received signal, and is supplied to a gate of a first MOSFET 108. A source of the first MOSFET 108 is connected to the ground potential 103, while a drain of the first MOSFET 108 is connected to the supply of the antenna voltage limiter circuit 104. Both the regulator unit 105 and a drain of a second MOSFET 110 are connected to said supply. A source of the second MOSFET 110 is connected to the ground potential 103. It should be noted that the MOSFETs are presumed to be of the N-channel type. Identifiers change accordingly when P-channel types are used.
Each value of the regulating signal Vreg causes a certain conductance of the first MOSFET 108 which in turn increases or decreases the supply voltage of the antenna voltage limiter circuit 104 and thus the supply voltage of the RFID tag 100 (including said “other portions” of an RFID tag which are omitted in FIG. 1). The higher the conductance, the lower the supply voltage.
The second MOSFET 110 is switched parallel to the first MOSFET 109. A modulator unit 109 is coupled to a gate of the second MOSFET 110 and provides the second MOSFET 110 with control signals to characteristically “short-circuit” the antenna coil 101 in a pulsed manner in order to modulate the antenna coil signal to send data. Variations in the electromagnetic field due to this modulation may be recognized by a read/write device so that the sent data may be retrieved from the modified electromagnetic field.
FIG. 2 is a diagram 200 illustrating an antenna voltage, or, in other words, an envelope 203 of the high-frequency signal. The diagram 200 has an abscissa 201 on which the time t is plotted. A voltage VLA/LB is plotted on an ordinate 202 of the diagram 200, wherein LA and LB denote the first coil connection 111 and the second coil connection 112, respectively. The diagram 200 shows a first portion 204 corresponding to a command-receiving mode, and a second portion 205 corresponding to a back-modulation operation mode.
In the system described with reference to FIGS. 1 and 2, the operation voltage VPEAK1 of the antenna voltage limiter circuit 104 is fixed to a value at which the command-receiving circuit (demodulator 107) starts working even at low electromagnetic field strengths using the voltage-limiting regulating signal Vreg. Since this level is maintained for the response of the RFID tag 100 as well, a relatively small voltage swing occurs at the coil connections during load modulation and thus also at the receiver of the read/write device. This may have the consequence that the recognition of the RFID tag 100 response by the reader is relatively prone to failure.